Dedicated Private Lines

In telephony, a private line or tie line is a service that involves dedicated circuits, private switching arrangements, and/or predefined transmission paths, whether virtual or physical, which provide communications between locations. In practice, dedicated, private lines may not be provided by a single, discrete, end-to-end cable, but they do provide guarantees of constant bandwidth availability and near-constant latency, properties that cannot be guaranteed for more public systems. Such properties add a considerable premium to the price charged.

The number of costly private lines used by commercial organizations is decreasing. They are being replaced by VPNs, which are less costly to maintain and have lower monthly lease rates. However, large enterprises, utilities, and financial services organization still use high-speed private lines for high-speed, secure communications.

Factoids about private line services:

• Pricing - Private, dedicated links are priced at flat monthly fees. The fees are not based on minutes used or the amount of data transmitted

• Fixed routes - Dedicated lines are not flexible. Calls and data can only be sent between the fixed points to which the lines are connected. Thus, communications with a site not on the network is not possible

• Exclusive use - Dedicated, private lines can only be used by the organization that leases them

• Multi-service - Dedicated lines are suitable for transmission of video, voice and data. Voice, video and data can share the same dedicated services, or they can use completely different dedicated lines. Firms often lease T-3 lines that have 672 channels to tie locations together. They can use, for example, 24 of the paths for voice and the rest for data and video

• Fixed capacity - Dedicated services are leased or built with a fixed capacity or bandwidth. These speeds range from low 9,600 bps up to OC-3 (155 mbps) and ATM megabit speeds. They also include T-1 and T-3 and fractional T-1 and T-3 speeds

• Security - Dedicated lines provide secure transmissions. Organizations concerned about security may place encryption devices on both ends of dedicated services. The encryption device scrambles the transmission when it leaves the sending location and unscrambles it when it arrives at the receiving location

• Convenience of services - Private lines provide abbreviated dialing and other convenient features, e.g., 4- or 5-digit dialing between sites, one operator can answer calls for multiple locations, one voice mail system can be shared by multiple locations, one directory across locations

Metropolitan area networks (MANs) consist of private lines that connect buildings within a city or metropolitan area. Large hospitals transmit customer records, research files, and radiology images over MANs. Major univserties also use MANs. Dedicated services are available around the clock. This is cost effective for companies that use the dedicated lines for voice, video, and e-mail during the day and bulk data transmissions after hours.